Liquid-liquid extraction has been used earlier in the metallurgical industry typically in the processing of solutions with a weak valuable metal content. Many large copper and uranium recovery extraction plants fall into this category. With regard to copper, however, the situation is changing, because the extraction feed solutions are becoming noticeably stronger with the pressurised concentrate leaching processes coming into operation. Likewise some cobalt and zinc extraction processes also treat strong feed solutions. Nevertheless, the size of the equipment, particularly in the case of copper, will generally remain large, also in the new pressurised leaching processes.
In all extraction processes a precious metal-containing aqueous solution is brought into contact with an organic solution in the extraction mixing section, forming a dispersion of two solutions that are insoluble in each other. The solutions in the dispersion are separated from each other in the separation section of extraction, where the solutions separate from each other into two layers with a dispersion band remaining between them. During the mixing stage either one or more of the valuable metals in the aqueous solution is transferred to the organic phase, from which the valuable metals are recovered by stripping the aqueous solution. Extraction is performed in an equipment, where the mixing and separation sections are either located one on top of the other (column) or in series on more or less the same level horizontally. Almost always in cases when large-scale extractions of weak solutions are concerned, such as copper extraction, the equipment is positioned in an essentially horizontal position. When we refer to extraction hereinafter, the term is used for various arrangements, but essentially equipment in the same level.
U.S. Pat. No. 6,132,615 describes a method and equipment for regulating the rate of extraction solutions in an extraction separation section. The method and equipment are based on several picket fences, of a form designated in the publication, located at the front end of the separation section. The picket fences extend across the settler to its sides. Vertically the picket fences extend up to above the liquid surface and without exception down to the bottom of the settler. The vertical control of the aqueous solution layer has been improved so that the dispersion band is made to extend as far as the farthest end of the separation section especially when the dispersion is of a water droplet dispersion type. The dispersion and the solution phases separated from it are forced to flow through a picket fence in at least three places in the separation section concerned. The picket fences are vertical slots of construction. The publication states that a strong dispersion band improves the performance values of the separation section in terms of both the quantity and quality of the separated solutions. A picket fence enables an increase of flow resistance in the separation section in the range of 250-600 Pa per fence. In a certain way shaped picket fences are a fairly good solution in many situations.
WO patent application 99/11830 describes a method and equipment for liquid-liquid extraction, where the dispersion coming from the mixer is fed into the bottom section of the settler and is directed from there firstly to a distribution channel located inside the settler. In the distribution channel the dispersion flows first in accordance with the main flow direction of the settler, but the direction of the flow is turned so that the dispersion is discharged into the actual settler space above the distribution channel, from the front part of the feed end.